Research Seminar: Tim Wahl

A research seminar from UNC Marine Sciences graduate student, Tim Wahl. Presented by the UNC at Chapel Hill Department of Marine Sciences. The location of this event will be in seminar room G201 on the ground floor of Murray Hall on UNC-CH campus in Chapel Hill, NC. This event will be held on Monday, December 3rd at 12:20pm.

Seminar Title: Temporal Variability of Dissolved Inorganic Nitrogen in Florida Bay following Hurricane Irma

Abstract: On September 10, 2017 Hurricane Irma passed over the Middle Florida Keys as a Category 4 storm with 130 mph winds causing extreme damage to both the environment and populated land areas. The hurricane caused water to recede approximately 200 m out from the Florida Bayside shoreline, resulting in catastrophic damage to nearshore benthic communities including sponges whose biomass decreased by up to 94% in shallow embayments. Other episodic events have produced such large-scale impacts: A 2013 phytoplankton bloom reduced sponge biomass by over 99% at a central basin site, Mystery Basin, located seven miles offshore into the Bay. The almost total removal of sponge biomass caused by this single bloom event drastically decreased their dissolved inorganic nitrogen (DIN) flux which had accounted for 48 ± 18 % of the total DIN flux to the Basin. A post-Irma time-series study featuring monthly water column sampling at 15 sites in southern Florida Bay, was established to understand how blooms and large storm events could influence temporal variability of DIN concentrations, particularly in central Florida Bay. The monthly whole water samples were analyzed for nitrate + nitrite (NOx), ammonium, dissolved organic carbon, total nitrogen, and chlorophyll-a. The study revealed quantitative DIN removal from the water column during a post-Irma phytoplankton bloom, however, following collapse of the bloom, concentrations rose close to pre-Irma levels and then sharply increased following a sediment resuspension event caused by a storm in December 2017. Injection of ammonium-enriched pore waters into overlying waters during the resuspension event appears to be the cause of this latter increase. Temporal changes of DIN and other parameters in the time-series sample suite revealed a return to pre-hurricane concentrations for these nutrients. However, the decimation of sponge biomass during blooms and Hurricane Irma plus newly recognized inputs of storm impacts will require new models for nitrogen cycling in Florida Bay waters, including the potentially larger role of storm perturbed sediments as a source of recycled DIN.